Ferromagnetic material



3,046,227 FERROMAGNETIC MATERIAL Evert Willem Gorter, Frederik Karel Lotgering, and Hugo Gerrit Blejers, all of Eindhoven, Netherlands, assignors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Oct. 20, 1958, Ser. No. 768,039 Claims priority, application Netherlands Oct. 21, 1957 11 Claims. (Cl. 252-625) Our invention relates to ferromagnetic materials and methods of making the same. In particular, our invention relates to ferromagnetic oxidic materials adapted for use in microwave equipment at extremely high frequencies.

Certain ferromagnetic oxidic materials, such as V B aFelzo g exhibit permanent properties and can be used at extremely high frequencies, i.e., -l5,00030,000 megacycles/sec. These materials, which are described in detail in US. Patent 2,762,777, have a crystal structure similar to that of the mineral magneto-plumbite, that is, they have a hexagonal structure with a c-axis of about 23.3 A. and an a-axis of about 5.9 A. The crystals of these materials exhibit a large magnetic crystal anisotropy in the direction of the hexagonal axis. Consequently, where use is made of the Faraday rotation or magnetic resonance, bodies constituted of these materials may be used for various microwave applications.

However, because of their large magnetic crystalline anisotropy which can be described by an effective anisotropy field of about 17,000 oersted, Faraday rotation occurs at l to 2 cms. (15,00030,000 mc./s.), and magnetic resonance occurs at about 6 mms. (50,000 mc./s.). If an external field were applied to the body, in addition to its own internal field resulting from the large magnetic crystal anisotropy, Faraday rotation and magnetic resonance occurs at shorter wave-lengths (higher frequencies). It is not possible to lower the frequency (increase the wave-length) at which these effects can occur with these materials.

On the other hand, ferromagnetic oxidic materials having a spinel crystal structure are also known. These materials are magnetically soft and hence have no in ternal field of consequence. These materials also exhibit Faraday rotation and magnetic resonance at frequencies (or wave-lengths) which depend upon the strength of an external field. As a practical matter, it is possible to realize external fields of such magnitude in micro-wave apparatus, e.g. a wave-guide, such that magnetic resonance occurs in those materials in a range from 3 to 15 crns. (2,000 to 10,000 mc./s.). Thus, a range of wave-lengths, e.g. 6 mms. to 3 cms., exists in which present materials cannot be used satisfactorily.

It is a principal object of our invention to provide a new ferromagnetic oxidic material in which magnetic resonance occurs in the range of 10,000 to 50,000 mc./s. (6 mms. to 3 ems.) requiring no external magnetic field.

It is a further object of our invention to provide a new ferromagnetic material for use at microwave frequencies which exhibits very small electrical and magnetic losses.

It is another object of our invention to provide a new ferromagnetic oxidic material which is magnetically anisotropic and which can be oriented in a preferred direction for use in microwave apparatus.

Another object of our invention is to provide a method of making ferromagnetic oxidic materials useful at microwave frequencies.

These and further objects of our invention will appear as the specification progresses.

We have found that ferromagnetic material consisting 3,046,227 Patented July 24, 1962 of hexagonal crystals similar to those of the compound BaFe O the crystals having a composition corresponding to the formula:

wherein D is a tetravalent ion of at least one of the metals Ti, Ge, Zr, Hf and Sn, wherein Me is a bivalent ion of at least one of the metals Mn, Ni, Zn, Mg and Cu, and wherein are particularly suited for use at frequencies of 10,000 to 50,000 mc./s. (6 mms. to 3 ems). The magnetic anisotropy of these crystals may be described by means of effective anisotropy fields from 16,000 oersted to low values in the direction of the hexagonal axis. Since the materials are oxidic, their specific resistance has a comparatively high value. High vmues for the specific resistance may occur more particularly in materials in which Me represents at least Cu and in those containing trivalent manganese (i=0).

In the applications of the above described materials use is naturally made of bodies built up of these materials. Since in certain cases it is desirable for the bodies to be magnetically anisoL opic, use in then made more particularly of bodies exhibiting a certain texture, that is to say bodies in which the particles are present in a condition relatively more or less oriented.

Bodies consisting of a ferromagnetic material according to the invention and more particularly those in which a certain texture is present may be used in transmission systems for microwaves. On account of the anisotropy field of these materials applications of magnetic resonance are possible in the range of from 6 to 30 mms. (10,000 to 50,000 mc./s.). This range is determined not only by anisotropy, but also by demagnetization. As with the above-described use of ferrites with spinel structure, when using the bodies according to the invention, the lower limit of the wave-length may be further decreased by applying an external magnetic field, the wave-length then being determined by the anisotropy field, the demagnetization and the external magnetic field.

The materials according to the invention are manufactured preferably by heating (sintering) a finely-divided and approximately correctly proportioned mixture of the component metal oxides of the new compounds. It is naturally possible for one or more of the component metal oxides to be replaced wholly or in part by compounds which can change to metal oxides upon heating, for example carbonates, oxalates and acetates. It is also possible for the component metal oxides to be substituted wholly or in part by one or more reaction products of two or more of the component metal oxides, for example BaFe O The term correct ratio is to be understood in this case to mean a ratio of the quantities of metals in the initial mixture equal to that in the materials to be manufactured. If desired, it is possible first to pre-sinter the finely divided initial mixture, to pulverize the reaction product and to sinter again the resultant powder. This series of treatments may, if desired, be repeated once or several times.

The temperature of the sintering or final sintering process, is chosen between about C. and about 1450" C., for example between 1200 C. and 1350 C. In order to facilitate the sintering process, it is possible to add sintering agents, such as silicates and fluorides.

Bodies made of the above-described ferromagnetic 3 materials may be manufactured by sintering the initial mixture of the metal oxides or the like right from the start in the desired shape and also by pulverizing the reaction product of the pre-sintering process and, if desired after the addition of a binding agent, forming it into the desired shape, followed by after-sintering.

Bodies which consist of the described ferromagnetic materials and which exhibit a certain texture, may be obtained by directing the particles of the ferromagnetic material, each particle having a preferential direction of magnetization, which can freely move to a certain extent with respect to one another, in a magnetic field to orient the particles with their preferential directions parallel to the principal direction of magnetization and fixing them into a coherent assembly as described in US. Patent 2,762,778. It is also possible for the particles fixed into a coherent assembly to be sintered to form a compact body. The powder preferably consists as far as possible of monocrystalline particles.

Alternatively, it is possible for bodies consisting of the described ferromagnetic materials and exhibiting a certain texture to be manufactured by including in the initial mixture at least one ferromagnetic compound, of which a body may be manufactured by means of a directing process. If the particles of this material are present in a relatively oriented condition, they can be directed in a magnetic field while still being freely movable to a certain extent with respect to one another. Upon heating, the mixed crystals are formed which are also magnetically oriented. In this case also, the powder preferably consists, as far as possible, of monocryst-alline particles as far as the said ferromagnetic compound is con cerned. This method affords the advantage that the directing process may be applied to particles having an anisotropy higher than that of the particles of which the ferromagnetic body to be manufactured is built up.

Since for certain applications it is preferable to use bodies having a comparatively high density, this has to be taken into account in the manufacture, for example by pulverizing the initial mixture and, if necessary, the body of the pre-sintering process to extreme fineness and sintering the body at a comparatively high temperature. However, the latter may have the disadvantage that a small proportion of the iron changes to the bivalent condition, so that the specific resistance of the body has a low value and possibly even an undesirably low value.

The following examples are illustrative of the invention which is defined with greater particularity in the appended claims.

EXAMPLE I A mixture consisting of BaFe O BaCO T10 and Coco in a ratio of 0.9 mol of BaFe O 0.1 mol 'of BaCO 0.6 mol of TiO- and 0.6 mol of CoCO which corresponds to the desired compound was ground with alcohol in a shaking mill for 8 hours. The ground product was suspended in acetone and a portion thereof was molded at a pressure of nearly 1 ton/ cm. into a tablet in a direct magnetic field, having a field strength of 1000 oersted parallel to the direction of molding. It is possible to apply a direction process to this mixture, since a body can be manufactured of the ferromagnetic compound BaFe O in which the particles are present in a relatively oriented condition. The tablet was heated from room temperature up to 500 C. within 16 hours and from 500 to 1210 C. within hours and subsequently sintered at 1210 C. in oxygen for 2 hours. The reaction may be represented by the equation:

( 913311 612010 BECO 6TiO 600 C03" 'l'he density of this tablet was 3.9 g./cm. and its specific resistance 2 l0 ohm cm. By means of a torsion method similar to that described in Physica 8, 562-565, 1941, the magnetic anisotropy of this tablet in the direction of the magnetic field during molding was determined. its value, expressed in an effective field H was 9800 oersted. By means of an X-ray diffractometer, it was ascertained, as in the examples following hereinafter, that the tablet was built up of particles having a crystal structure similar to that of the compound BaFe O and that the particles were present in the body in a relatively almost completely oriented condition.

A small plate of 10 x 30 x 0.15 mm. was cut firom the tablet so that the side of 3 mms. was parallel to the direction of the magnetic field during the molding of the tablet. This plate was glued on a small trapezium-like quartz plate of 0.7 mm. thick. The whole was arranged in a rectangular wave guide of 7.1 x 3.55 mms. in parallel to the short side-wall having a height of 3.55 mms.

In a direction at right angles to the ferromagnetic plate, there was applied a magnetic field H which was adjusted to a maximum absorption of energy in the backward direction at a frequency of 35,000 mc./s. Subsequently, the distance between the plate and the short side wall was varied in the presence of the field l-I until the damping in the direction of passage was minimum, which was the case with a distance of about 0.6 mm. The said damping is the damping of the microwaves in the direction of propagation. Then the damping ratio d was determined,

that is the ratio between the damping of the microwaves in the direction opposite to the direction of propagation and the damping of the microwaves in the direction of propagation. a was 12.0 and H was 700 oersted.

Tablets were manufactured in a similar manner according to the reaction equation specified hereinafter, the tablets being sintered at the temperatures indicated in the table. The density, the specific resistance and the effective anisotropy field H of these tablets were determined. A small plate was cut from several of these tablets in the indicated manner and tested in the abovedescribed way. The values of the damping ratio d and of the applied magnetic field H are also specified in the table.

Table temp. denspec HA H p No. Composition sint., sity, resist Oers. dv Oer.

0. g./em. 52cm BEtT'iorCOosFGiMOrQ 1, 210 3. 9 2. 1O 9, 800 700 BaTlo.sZlloaFemshinojzoia 1, 330 5. 5. 10 9, 200 1, 760 4. 1 3. 10 8. 200 0 4. 3 8. 10 8, 000 0 4. 2 3. 5, 000 aT 1.2Cin .2F99n019 1, 275 4. 6 2. 10 7, 600 BaGe1.2 01.2 e9.e01o g g 5. 1 7. 10 7, 100 5. 1 3. 10 10,000 4. 3 5. 10 6, 780 4. 3 3. 10 5, 330 4. 2 5. 10 6, 900 4. 1 3. 1O 4, 500

While we have described our invention in connection with specific embodiments and applications, other modifications thereof will be readily apparent to those skilled in this art without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. A ferromagnetic body particularly adaptedfor use in microwave apparatus consisting essentially of a coherent mass of mixed crystals of compounds having a hexagonal crystal structure similar to that of the compound BaFe120l9, the composition of said mixed crystals corresponding to the formula in which I) is a quadrivalent metal ion selected from the group consisting of Ti, Ge, Zr, Hf, and Sn, Me is a bivalent metal ion selected from the group consisting of Mn, Ni++, Zn++, Mg, and Cu, and in which a, b, arid e are not more than 1, c is not more than 0.4, d is between 0.4 and 2.5, and f not more than 3, said crystals having a preferred direction of magnetization and being oriented in said preferred direction whereby said body is magnetically anisotropic and resonant at wavelengths of about 6 mms. to 1 cm.

2. A ferromagnetic body particularly adapted for use in microwave apparatus consisting essentially of a coherent mass of mixed crystals of compounds having a hexagonal crystal structure similar to that of the compound BaFe O the composition of said mixed crystals. corresponding to the formula (labc) a b c d gflz) g;-2df) l9 in which D is a quadrivalent metal ion selected from the group consisting of Ti, Ge, Zr, Hf, and Sn, and in which a, b, and e are not more than 1, c is not more than 0.4, d is between 0.4 and 2.5, f not more than 3, said crystals having a preferred direction of magnetization and being oriented in said preferred direction whereby said body is magnetically anisotropic and resonant at wave-lengths of about 6 mms. to 1 cm.

3. A ferromagnetic material consisting essentially of hexagonal crystals whose structure is similar to those of BaFe O and having a composition:

B (lab-c) a b c d (d-e) t a l ;-2di) i l9 in which D is a quadrivalent metal ion selected from the group consisting of Ti, Ge, Zr, Hf, and Sn, Me is a bivalent metal ion selected from the group consisting of Mn++, Ni++, Zn++, Mg, and Cu, and in which a, b, and e are not more than 1, c is not more than 0.4, d is between 0.4 and 2.5, and f not more than 3.

4. A ferromagnetic material consisting essentially of hexagonal crystals whose structure is similar to those of BaFe O and having a composition:

B a (lab-o) a b 'c d E-e) i iii-mm Di w in which D is a quadrivalent metal ion selected from the group consisting of Ti, Ge, Zr, Hf, and Sn, and in which a, b, and e are not more than 1, c is not more than 0.4, d is between 0.4 and 2.5 and f not more than 3.

5. A ferromagnetic body particularly adapted for use in microwave apparatus consisting essentially of a coherent mass of hexagonal crystals whose structure is similar to those of BaFe m, ach of said crystals having a preferential direction of magnetization and .a composition:

in which D is a quadrivalent metal ion selected from the group consisting of Ti, Ge, Zr, Hf, and Sn, Me is a bivalent metal ion selected from the group consisting of Mn Ni, Zn, Mg++, and Cu++, and in which a, b, and e are not more than 1, c is not more than 024, d is between 0.4 and 2.5, and f not more than 3, said body being magnetically anisotropic in a principal direction and resonant at wave-lengths of about 6 mms. to 1 cm., said crystals being oriented with their preferential directions of magnetization parallel to said principal direction.

6. A ferromagnetic body particularly adapted for use in microwave apparatus consisting essentially of a coherent mass of hexagonal crystals whose structure is similar to those of BaFe each of said crystals having a preferential direction of magnetization and a composition:

in which D is a quadrivalent metal ion selected from the group consisting of Ti, Ge, Zr, Hf, and Sn, and in which a, b, and e are not more than 0.4, d is between 0.4 and 2.5, and f not more than 3, said body being magnetically anisotropic in a principal direction and resonant at wave-lengths of about 6 mms. to 1 cm., said crystals being oriented with their preferential directions of magnetization parallel to said principal direction.

7. A method of manufacturing a ferromagnetic material comprising the steps, forming a finely-divided mixture of a material having a composition:

the values of a and 19 being not more than 1, 0 being not more than 0.4, an om'de of a metal selected from the group consisting of Mn, Ni, Zn, Mg, and Cu, an oxide of a metal selected from the group consisting of Ti, Ge, Zr, Hf, and Sn, and cobalt oxide, all in correct proportions and yielding upon firing hexagonal crystals similar to those of said material and having a composition corresponding to the formula:

B3, SI Pb Ca D Me Co FeE 2 MnP Om in which D is a quadrivalent metal ion selected from the group consisting of Ti, Ge, Zr, Hf, and Sn, Me is a bivalent metal ion selected from the group consisting of Mn++, Ni++, Zn++, Mg++, and Cu++, the values of a, b and 2 being not more than 1, c being not more than 0.4, d is between 0.4 and 2.5, and f not more than 3, and heating the same in an atmosphere consisting essentially of oxygen at a temperature of about 1000 to 1450 C. to form said crystals.

8. A method of manufacturing a magnetically anisotropic body having a principal direction of magnetization and adapted for use at microwave frequencies as defined in claim 7 in which said finely-divided mixture is oriented in a magnetic field so that the preferential directions of magnetization of the crystals produced upon heating are parallel to said principal direction of magnetization in said body, and thereafter said finely-divided mixture is compacted into a body and heated at said temperature to form said crystals.

9. A method of manufacturing a magnetically anisotropic body having a principal direction of magnetization and adapted for use at microwave frequencies in which the material obtained in accordance with the method of claim 7 is finely-divided, the particles each having a preferential direction of magnetization, orienting the particles with their preferential directions of magnetization parallel to said principal direction of magnetization of the body, and compacting the oriented particles into a coherent body.

10. A method of manufacturing a magnetically anisotropic body adapted for use at microwave frequencies as defined in claim 9 in which the particles are magnetically oriented.

1:1. A method of manufacturing a magnetically anisotropic body adapted for use at microwave frequencies as defined in claim 10 in which the compacted body is sintered.

References Cited in the file of this patent UNITED STATES PATENTS 2,640,813 Berge June 2, 1953 2,656,319 Berge Oct. 20, 1953 2,659,698 Berge Nov. 17, 1953 2,736,708 Crowley et a1. Feb. 28, 1956 2,762,777 Went et al. Sept. 11, 1956 2,762,778 Gorter et al. Sept. 11, 1956 2,837,483 I-Iakker et al. June 3, 1958 2,847,101 Bergman Aug. 12, 1958 FOREIGN PATENTS 516,395 Belgium June 19, 1953 735,375 Great Britain Aug. 17, 1955 1,110,334 France Oct. 12, 1955 211,542 Australia Nov. 25, 1957 OTHER REFERENCES 

1. A FERROMAGNETIC BODY PARTICULARLY ADAPTED FOR USE IN MICROWAVE APPARTUS CONSISTING ESSENTIALLY OF A COHERENT MASS OF MIXED CRYSTALS OF COMPOUNDS HAVING A HEXAGONAL CRYSTAL STRUCTURE SIMILAR TO THAT OF THE COMPOUND BAFE12O19, THE COMPOSITION OF SAID MIXED CRYSTALS CORRESPONDING TO THE FORMULA 